1. Field of the Invention
This invention provides an enhanced rotary face seal design for roller cone rock bits. The new seal has a single energizer and a geometry which allows self adjusting volume balance compensation in bits with floating journal bearings.
2. Description of the Related Art
Modern, premium roller cone rock bits utilize sealing systems to prevent the loss of lubricant from the roller cones. The seal system also prevents the abrasive laden drilling fluid outside the bit from entering into, and causing failure of the bearing system of the rolling cones.
In most drill bits, an elastomeric packing ring provides the seal between the rolling cone and the bearing system. These bits utilize an elastomeric compression type sealing system, and have adequate performance in most drilling applications. For rock bits used in very severe bit applications, however, rotary mechanical face seals are disposed between the rolling cone and the bearing to provide the seal.
Rotary mechanical face seals are generally made up of two flat sealing faces which are designed to maintain a thin film of lubricant between the sealing faces. As the sealing surfaces rotate relative to each other, they are urged together at a carefully controlled force by one or more energizers as shown, for instance, in U.S. Pat. Nos. 5,360,076, 4,306,727, and 3,761,145.
Although generally more expensive than elastomer seals, mechanical face seals are able to assure a level of performance in rock drilling bits which easily justifies the higher cost. Most mechanical face seals used in rotary rock bits are made from stainless steels and have sealing faces which are manufactured to be flat and smooth. These faces mate together to form a planar, annular sealing interface.
Mechanical face seals (also known as rigid face seals) have become the seal of choice for rock bits used in the most severe drilling environments, due to the operating limitations of elastomers as dynamic seals. The mechanical face seals are typically manufactured from materials which readily tolerate the thermal, chemical and mechanical attack of severe drilling environments. The seals provide a higher level of reliability than elastomer seals in rock bits and are capable of extremely long runs without significant loss of lubricant.
A very important development for rigid face seals in rolling cutter drill bits is described in U.S. Pat. No. 4,516,641, herein incorporated by reference. When the bit is drilling, the bearing clearances between the rolling cutter and bearing spindle allow the cutter to move axially along the bearing shaft. This movement causes volume changes in the bearing area immediately adjacent to the seal. The magnitude of these volume changes depend upon the relative diameters of the journal bearing, the effective sealing diameters of the seal, and the axial movement of the cutter. These volume changes can cause extremely high local pressure spikes in the lubricant adjacent to the seal. Prior to the '641 invention, face seal designs for rolling cutter drill bits either allowed lubricant to vent between the seal faces during pressure spikes, or allowed the seal faces to endure extremely high loading during the pressure spikes.
In the '641 patent, the rigid face seal was made to be volume compensating. This allowed the seal to sweep a volume corresponding to the volume change of the lubricant in the cutter adjacent to the seal. The ability of the seal to move in response to volume changes eliminated the extreme pressure spikes adjacent to the seal during operation. In addition, volume compensation allows the sealing face load on the seal to remain relatively constant as the cutter moves axially upon the bearing journal during drilling. This design, therefore, provided a reliable rigid face seal for rock bits that could run for very long hours without significant loss of lubricant.
A limitation of this design, however, is the tendency of the static elastomeric seals/energizers to permanently deform in service. Relatively high seal squeeze on the elastomer is needed for the seal/energizer to transmit the torque of the seal faces. As is well know in the elastomer industry, elastomers tend to take a compression set in service. Once the elastomer has taken a set in service, slippage of the seal ring becomes more likely. Even a small amount of slippage can lead to rapid, catastrophic failure of the seal system.
For some bit designs, it is possible to design a bearing and seal combination that has little or no net lubricant volume displacement in the area of the bearing journal adjacent to the seal as the cutter moves axially. One such design is shown in U.S. Pat. No. 4,573,304, herein incorporated by reference. In this rigid face seal design, the pressures and volume are balanced by placing the static sealing element so that its mean diameter is the same as the diameter of the bearing journal. This design is also susceptible to seal/energizer slippage in service, leading to catastrophic seal failure.
Until the present invention, a single energizer, volume balanced rigid face seal design was not possible for drill bits that use a separate floating journal bearing. As shown for instance in U.S. Pat. No. 4,565,800, the effective diameter of the bearing journal is indeterminate. The effective diameter could be either the inside or outside diameter of the bearing, depending on whether it is rotating with the cutter or stationary on the journal. To compensate for this uncertainty, the axial play of the cutter upon the bearing journal is carefully limited, thus limiting the volume change.
Single energizer mechanical face seals are old in the drill bit industry. In addition to the single energizer mechanical face seals cited thus far in this specification, other single energizer mechanical face seals for rolling cutter drill bits are shown in U.S. Pat. Nos. 4,838,365; 4,824,123; 47,847,604; and 4,172,502. These patents utilizing rigid and/or compliant seal faces represent different types of single energizer mechanical face seals for rolling cutter drill bits. Many of these designs have had success in rolling cutter drill bits. However, none have enjoyed widespread commercial success in drill bits with floating journal bearings.